In radio-frequency (RF) communications, multiple-input-multiple-output (MIMO) refers to using multiple antennas at both the transmitter and the receiver ends to enhance communication performance. The terms input and output in MIMO denote the radio channels that carry the input and output signals received by or transmitted from a communication device (e.g., a transceiver). MIMO, which is an important portion of modern wireless communication standards such as IEEE 802.11n (e.g., Wi-Fi), 4G, 3GPP Long Term Evolution (LTE), IEEE 802.16e (e.g., WiMAX), and others, can significantly increase data throughput and link range without the need for additional bandwidth or increased transmission power. Three main categories of MIMO include spatial multiplexing, diversity coding, and precoding.
MU-MIMO, which is part of the IEEE 802.11ac standard, allows a beamformer (e.g., a router such as an access point) to simultaneously transmit multicast data to more than one beamformees (e.g., stations such as an iPhone). In contrast to single user (SU-MIMO), where at a given time on a given channel the access point is only able to communicate to one station, the data rate is usually limited by the number of receive antennas at the station (e.g., due to size limitations of a mobile device). MU-MIMO, on the other hand, is able to fully utilize the spatial degrees of freedom and allows the access point to communicate to more than one station on the same channel at the same time. This could potentially scale the system throughput by N folds, where N is proportional to the number of transmit (TX) antennas of the access point divided by the number of receive (RX) antennas on the station. MU-MIMO technology is developed based on a set of frame exchange protocols which allows the beamformer (e.g., access point) and the beamformees (e.g., stations) to keep high data transmission without collisions. However, it's also costly to follow the existing primitive form of the frame exchange protocol that is highly inefficient.